Crude phthalic anhydride contains a number of lower- and higher-boiling byproducts such as benzoic acid, maleic anhydride, phthalide, etc., which for reasons of the requirements as to the quality of the pure phthalic anhydride must be removed as far as possible. Up to now, crude phthalic anhydride is liberated from lower-boiling byproducts in a first distillation stage. In a second distillation stage, high-boiling components are separated, and pure phthalic anhydride is recovered as top product.
The distillation of the crude phthalic anhydride has so far been performed as follows: As a result of gravimetric level differences, the crude phthalic anhydride coming from the thermal pretreatment (where water is removed and/or phthalic acid is converted to phthalic anhydride) gets into the so-called pretreatment column, where the crude phthalic anhydride is introduced in liquid form between the uppermost tray and the overlying structured packing and is expanded into the column. In the bottom of the pretreatment column, the crude phthalic anhydride is evaporated. The ascending vapors undergo a mass transfer with the liquid trickling down from the inlet of crude phthalic anhydride or of the top condenser. From the top condensate, in which the lower-boiling byproducts (together with phthalic anhydride) are concentrated, a small part is withdrawn and delivered as waste to the so-called residue container. Thus, the bottom in the pretreatment column is liberated from low-boiling components. The boiling liquid in the bottom is gravimetrically supplied to the bottom of the pure-phthalic-anhydride column. This is effected by means of an overflow. This column operates as pure enriching column and is equipped with 20 valve trays. At the top of the pure-phthalic-anhydride column the pure phthalic anhydride is withdrawn and in part supplied as reflux to the upper tray. The other part is withdrawn as pure product. The bottom residue of the column (high-boiling components) is withdrawn from the column and discontinuously heated to about 250° C. in a boiling vessel, so as to recover phthalic anhydride contained therein by evaporation. Performing the distillation by means of such apparatus has been quite successful for decades, but has the disadvantage that two complete columns are required. This involves not only high investment costs, but also the necessary space requirement for the associated steel construction. Both columns operate under a vacuum of about 140 mbar and have separate vacuum systems.
Columns for concentrating phthalic anhydride are described for instance in DE-A-3538911. There are used two distillation columns arranged side by side for producing e.g. phthalic anhydride. EP-B-0087678 describes a very high tray column, likewise for purifying phthalic anhydride. There are likewise known columns which by means of a partition are divided into partial regions of the column, in order to avoid the cross-mixing of liquid or vapor streams. This is described in DE-A-3522234. In DE-A-4336986 a column is described, which is divided over the entire cross-section by means of a baffle plate. In this column, one mass transfer package each is arranged in both column halves. Both columns are not suitable for producing phthalic anhydride.
Proceeding from this prior art, it is the object underlying the invention to improve the previous column arrangement in terms of apparatus and make it less expensive and easier to install.